1. Technical Field
The present invention relates to electronic microcircuits and specifically to methods and apparatus for the detection of metal extrusion associated with electromigration in high-current-density settings.
2. Background of the Invention
Metal extrusion arising from electromigration, though known prior to the development of modern integrated circuits in the 1960s, did not present challenges to the design of electronic microcircuitry until metal connectors, or interconnects, became so small that current densities on the order of 106 amps/cm2 were common. Such large current densities cause heating of the metal interconnects, but most of the heat is rapidly conducted away by adjacent thermally conductive substrates and surfaces. Still, such high current densities and component temperatures can induce electromigration processes that can, in turn, adversely affect circuit reliability which is essential to the microelectronics industry wherein products typically must work for more than 10 years. Short service life is useful only in such short-lived applications as missile guidance systems. If the probability of failure of say a transistor is one in a million per year, then failure is a certainty in an IC comprised of a million transistors. And since modern ICs often contain more than 10 million circuit elements, acceptable reliability on the chip level must exceed one chance in a billion over a one-year period.
The physical basis of electromigration of metal atoms in an electrical conductor is related to the momentum exchange between conducting electrons and diffusing metal atoms in high-current situations. At any temperature above 0 K, atomic vibrations occur. These vibrations (“phonons”) put a metal atom out of its perfect position about 1013 times each second and disturb the periodic potential, causing electron scattering. The scattering event makes the electron change direction, i.e., undergo acceleration for which there is a corresponding force. After many collisions (another word for the scattering event), the force averages out in the direction of electron flow. The force due to collisions of electrons to metal atoms is called the momentum exchange, which is the same as force. To provide sufficient momentum exchange to cause measurable effects, many electrons must be available to collide with the atoms. This can only happen in metals because many electrons are easily accelerated within an imposed electric field. Also, semiconductors have far fewer electrons and, in a true semiconductor, electromigration does not exist because there are insufficient charge carriers. However, electromigration can occur in semiconductor-like materials, such as silicon when they are so heavily doped as to act as if it were a metal. At dopant levels of around 1%, electromigration has been seen in polycrystalline silicon when the temperature coefficient of resistance (TCR) is positive. A positive TCR is probably a good definition of a metal.
The greatest momentum exchange occurs only at the sites where it is possible for atoms to move. Simply stated, electrons collide with metal atoms and produce a force in the direction of electron flow (for n-type materials, opposite for p-type materials). In general, the electromigration force is proportional to the current density.
Typically, electromigration-induced metal extrusion occurs at the anode end of an interconnect to which electrons and thus metal atoms flow, causing delamination/cracking in dielectric layers and eventually electrical shorting to adjacent current-carrying lines.
One early solution to the electromigration problem was to use conductors that were resistant to electromigration by alloying the aluminum with up to 4% copper. Due to processing considerations, the amount of copper was decreased to about 0.5%. However, the electromigration problem persists as IC technology is pushed to ever higher component densities. Electromigration is also considered a potential reliability concern in copper interconnects, even though copper is less susceptible to electromigration failure.
In more recent times, a standard method for detecting extrusion failures arising from electromigration has been by the use of “extrusion monitors” that are situated close to (<1 μm) electromigration (EM) test lines. As the extrusion occurs in the EM test line during electromigration stress associated with high current densities, an electrical short or increase in leakage current is expected to occur between the EM test line and the parallel-running extrusion monitor wire or lead. In reality, however, this is often not the case, i.e., metal extrusion can occur without causing electrical contact with the extrusion monitor, and thus extrusion cannot be detected by this leakage current method.
The known prior art relates to general electromigration measurement. The following are some typical examples of relevant patents:
U.S. Pat. No. 5,264,377, which describes the wafer-level electromigration’ test system, or so-called “SWEAT” test for fast in-line electromigration reliability monitoring. It does not mention detecting extrusion.
U.S. Pat. No. 5,514,974 proposes a chain of metal segments to accurately flag metal failure. This patent does not involve extrusion detection techniques, and thus is not relevant.
U.S. Pat. No. 6,598,182 ('182) describes a electromigration test system capable of real-time test monitor of metal resistance change as well as metal extrusion during electromigration stress. In other words, the extrusion monitor technique described in '182 is the standard detection method in which leakage current between the EM test line and the extrusion monitor wire is measured. Also, '182 discloses only the equipment to test the traditional structure, and it does not involve new structures or methods that are relevant to the present invention described in detail hereinbelow.